Deformation sensing can be achieved by placing point sensors across a certain range. However, this raises a problem when large engineering projects require the sensing to be done over several kilometers because numerous point sensors are required.
Conventionally, a distributed sensor is a device with a linear measurement basis, which is sensitive to a measure and at any of its points. Distributed optical fibre sensing is not well known and has been slow to be accepted into conservative large engineering projects where long sensors would be advantageous. The optical fibre is sensitive over its entire length. A single distributed optical fibre sensor can replace thousands of discrete point sensors. Traditionally, optical fibre connections were thought to be costly and troublesome. However, the cost of using fibre optics has fallen rapidly. Use of optical fibres is advantageous because they are tough, durable, stable, and can be applied in harsh environments. The fibres are also immune to electrical interference common in industrial environments and have small cross-sections, making them suitable for embedment in composite materials.
There are different types of optical fibre distributed sensors—those that measure temperature distributions by detecting Raman scattered light in a fibre, others that measure strain distributions by detecting Rayleigh scattered light, and still others that measure both temperature and strain distributions by detecting Brillouin scattered light. The sensors that are based on measurement of Brillouin scattered light include BOTDA (Brillouin Optical Time Domain Analysis), BOTDR (Brillouin Optical Time Domain Reflectometry), BOFDA (Brillouin Optical Frequency Domain Analysis) and correlation-based Brillouin distributed sensors.
A BOTDA sensor applies Brillouin Scattering, a method of detecting distributed temperature and strain using a non-linear optical effect. Generally, fibre strain and temperature are linearly associated with the frequency shift and hence the wavelength of light, caused by scattered light. Both strain and temperature cause a shift in the Brillouin frequency. The BOTDA sensor measures changes in the local strain and/or temperature conditions of an optical fibre through analysis of the Brillouin frequency of the fibre at any point. Position is determined by the round-trip transit time of the optical signal in the fibre, which is approximately 0.1 m/ns in typical fibres.
Typical fibres exhibit coefficients of change in Brillouin frequency Cε0.05 MHz per ppm change in length (microstrain, με) and CT≈1 MHz per ° C. change in temperature. The Brillouin frequency (vB) at a point z is therefore given by:vB(z)=vB0(z)+Cε·ε(z)+CT·T(z)  Eq.1where vB0(z) is the reference Brillouin frequency and T(z) and ε(z) are the local temperature and strain conditions respectively.
Typical BOTDA sensors can resolve around 1 MHz changes in Brillouin frequency resulting in a strain resolution of about 20 με or a temperature resolution of about 1° C. Since both temperature and strain affect the Brillouin frequency in the same way, it is normally impossible to identify which parameter has changed without further information or assumption (for instance, an assumption that the sensor is isothermal, or knowledge that the fibre is strain-free).
Some prior art sensors use a single strand of optical fibre. This is problematic since the Brillouin frequency is dependent on both local strain and temperature variables. Therefore, two strands of sensing fibre are often used in proximity of each other and placed in parallel—one detects strain and temperature, and the other detects temperature only. The fibre that detects temperature only is situated in a mechanically isolated tube to replicate a strain-free environment. Calculations of Brillouin frequency using such a set-up are inaccurate, however, since they are made with the assumption that the temperature is the same for both fibres; however, in reality, it is common for the temperatures to differ. In addition, even when the temperatures of the fibres are the same, thermal expansion of the host material will cause additional temperature-dependant strain that is not compensated for by the temperature-only fibre.
Other prior art sensors comprise at least two optical fibres in a single substrate with one of them measuring strain and temperature, and another measuring temperature only. Although this increases the likelihood that the fibres experience the same temperature conditions, thermal expansion can cause additional strain in the strain-measuring fibre that is not compensated for by the temperature-measuring fibre. In addition, these devices place the strain-sensing fibre along the neutral axis of the substrate and therefore cannot measure the curvature or displacement of the substrate itself.